1. Field of the Invention
The present invention relates to a negative-type planographic printing plate precursor which can be formed into a printing plate by a direct plate-making process using an infrared laser or the like based on digital signals of a computer, etc., i.e., so-called Computer-to-Plate (CTP) process.
2. Description of the Related Art
In recent years, lasers have been developed remarkably, and high-power small-size lasers are widely available. These lasers are very useful as a recording light source to be used for a direct printing plate-making (Computer-to-Plate: hereinafter, referred to as CTP) based on digital data from a computer or the like. In particular, a solid state laser and a semiconductor laser that emit infrared rays having wavelengths in a range of 760 nm to 1200 nm are particularly useful because of higher power in comparison with other wave-length ranges. Recently, there have been strong demands for an image-recording material having high sensitivity to an infrared laser, that is, an image-recording material whose solubility to a developer changes greatly upon irradiation with an infrared laser.
With respect to a negative-type image recording layer used in such a planographic printing plate precursor, public attention has been focused on those image recording layers in which, upon exposure, active species are generated in the image recording layer and the function thereof causes a physical or chemical change to make only the exposed portion insoluble so that the image recording layer at the unexposed portion is removed through the successive developing process to prepare a planographic printing plate. Among these, an image recording layer containing a light-to-heat conversion agent such as an IR absorber, a polymerization initiator that generates active species upon being heated by the light-to-heat conversion agent, a polymerizable compound such as an addition polymerizable ethylenic unsaturated compound that is subjected to a curing reaction by the active species, and further a binder polymer soluble to an alkaline developer are considered to be preferably used from the viewpoints of superior productivity and easiness in developing process.
Conventionally, with respect to the binder polymer constituting the photosensitive layer, organic high polymers capable of alkali developing, such as a methacrylic acid copolymer, an acrylic acid copolymer, an itaconic acid copolymer, a crotonic acid copolymer, a maleic acid copolymer and a partially esterified maleic acid copolymer, have been used (for example, see Japanese Patent Application Laid-Open (JP-A) No. 59-44615, Japanese Patent Application Publication (JP-B) Nos. 54-34327, 58-12577, 54-25957, JP-A Nos. 54-92723, 59-53836 and 59-71048).
However, in the case of a conventional planographic printing plate precursor having an image recording layer containing such a binder polymer, in the image area cured by exposure, there is an insufficiently cured area to which the developer permeates, resulting in problems of defective image portions and degradation in the printing press life.
In an attempt to prevent these problems, when a compound for suppressing permeability of the developer to the image area is added, the permeability of the developer to the non-image area is lowered to cause degradation in the developing property, with the result that stains tend to occur due to residual films in the non-image area. Consequently, it is very difficult to satisfy both the suppression of permeation of the developer in the image area and the high developing property in the non-image area.